In a distributed antenna system (DAS), antennas are distributed to and disposed in various locations within a cell, and a single base station (BS) manages these antennas. A DAS is different from the conventional centralized antenna system (CAS) in which antennas localize on the center of a cell.
A DAS is differentiated from a femto cell/pico cell, a relay station, and an ad-hoc system. The DAS is different from a femto cell/pico cell in which an individual antenna constitutes a separate network, in that an individual antenna does not administer an antenna region thereof by itself but a central BS administers all the antenna regions. Also, the DAS is discriminated from a relay station or an ad-hoc system, in that respective antennas may be connected by a fixed line. Also, the DAS is differentiated from a simple repeater, in that respective antennas may transmit different signals according to a command from a BS.
An initial purpose of a DAS was to install more antennas within a cell and repeatedly transmit signals in order to cover a shadow area. Namely, initially, a DAS was mainly aimed at securing coverage. Macroscopically, a DAS may be considered as a type of multi-input multi-output (MIMO) system in that antennas may simultaneously transmit or receive a plurality of data streams to support one or a plurality of users. A MIMO system is recognized as a system satisfying next-generation communication due to high spectral efficiency. In the point of view of a MIMO system, a DAS has more advantages than that of a CAS. For example, a distance between users and antennas is reduced to have high power efficiency, a channel capacity is increased due to low correlation and interference between antennas, and relatively uniform quality of communication can be secured irrespective of a user location within a cell.
Namely, a DAS having the foregoing advantages may be advantageous for implementing a MIMO system in order to secure a high data capacity required in the present and future communication standards, and is required to perform MIMO transmission. For example, it may be required to perform rank-2 transmission or higher to a single user equipment (UE) (it is known as a single user (SU)-MIMO) in the same frequency domain. Or, a multi-user (MU)-MIMO simultaneously supporting multiple UEs may be required. Such necessity may also be required in uplink, as well as in downlink.
The foregoing SU-MIMO and MU-MIMO are essentially required in IEEE 802 and 3GPP LTE as standardization groups, and in actuality, standards such as IEEE (Institute of Electrical and Electronics Engineers) 802.16e/m, 3GPP (3rd generation partnership project) LTE (long term evolution) Release 8/9, and the like, deal with SU-MIMO and MU-MIMO. However, currently, communication standards are designed on the premise of a CAS, so they cannot be applied as is to a DAS employing an advanced technique such as a MIMO technique. Thus, a communication standard supporting a DAS is required, and also, a method for transmitting a signal of a terminal (or a user equipment (UE)) based on such a communication standard and a UE using the method are required.